SN 1572 (Tycho's Supernova, Tycho's Nova), "B Cassiopeiae" (B Cas), or 3C 10 was a supernova of Type Ia[1] in the constellation Cassiopeia, one of about eight supernovae visible to the naked eye in historical records. It burst forth in early November 1572 and was independently discovered by many individuals.[2]

Star map of the constellation Cassiopeia showing the position (labelled I) of the supernova of 1572; from Tycho Brahe's De nova... stella

The appearance of the Milky Way supernova of 1572 belongs among the more important specific observation events in the history of astronomy. The appearance of the "new star" helped to revise ancient models of the heavens and to speed on a revolution in astronomy that began with the realized need to produce better astrometric star catalogues (and thus the need for more precise astronomical observing instruments). It also challenged the Aristotelian dogma of the unchangeability of the realm of stars.

The supernova of 1572 is often called "Tycho's supernova", because of Tycho Brahe's extensive work De nova et nullius aevi memoria prius visa stella ("Concerning the Star, new and never before seen in the life or memory of anyone", published in 1573 with reprints overseen by Johannes Kepler in 1602, and 1610), a work containing both Tycho Brahe's own observations and the analysis of sightings from many other observers. Tycho was not even close to being the first to observe the 1572 supernova, although he was probably the most accurate observer of the object (though not by much over some of his European colleagues like Wolfgang Schuler, Thomas Digges, John Dee, Francesco Maurolico, Jerónimo Muñoz,[3]Tadeáš Hájek, or Bartholomäus Reisacher[4]).

In England, Queen Elizabeth called to her the mathematician and astrologer Thomas Allen, "to have his advice about the new Star that appeared in the Cassiopeia to which he gave his Judgement very learnedly", as the antiquary John Aubrey recorded in his memoranda a century later.[5]

The more reliable contemporary reports state that the new star itself burst forth sometime between November 2 and 6, in 1572, when it rivalled Venus in brightness. The supernova remained visible to the naked eye into 1574, gradually fading until it disappeared from view.

The red circle visible in the upper left part of this WISE image is the remnant of SN 1572.

The distance to the supernova remnant has been estimated to between 2 and 5 kpc (approx. 6,500 and 16,300 light-years), but recent studies suggest a value closer to 2.5 and 3 kpc (approx. 8,000 and 9,800 light-years).[6]

The search for a supernova remnant was negative until 1952, when Hanbury Brown and Hazard reported a radio detection at 158.5 MHz, obtained at the Jodrell Bank Observatory.[7] This was confirmed at wavelength 1.9 m by Baldwin and Edge (1957),[8] and the remnant was also identified tentatively in the second Cambridge radio-source catalogue as object "2C 34" and identified more firmly as "3C 10" in the third Cambridge list (Edge et al. 1959). There is no dispute that 3C 10 is the remnant of the supernova observed in 1572–1573. Following a review article by Minkowski (1964),[9] the designation 3C 10 appears to be that most commonly used in the literature when referring to the radio remnant of B Cas (though some authors use the tabulated Galactic designation G120.7+2.1 of Green 1984, and many authors commonly refer to it as "Tycho's supernova remnant". Because the radio remnant was reported before the optical supernova-remnant wisps were discovered, the designation 3C 10 is used by some to signify the remnant at all wavelengths.

The supernova remnant of B Cas was discovered in the 1960s by scientists with a Palomar Mountain telescope as a very faint nebula. It was later photographed by a telescope on the international ROSAT spacecraft. The supernova has been confirmed as Type Ia,[1] in which a white dwarf star has accreted matter from a companion until it approaches the Chandrasekhar limit and explodes. This type of supernova does not typically create the spectacular nebula more typical of Type II supernovas, such as SN 1054 which created the Crab Nebula. A shell of gas is still expanding from its center at about 9,000 km/s. A recent study indicates a rate of expansion below 5,000 km/s.[10]

In October 2004, a letter in Nature reported the discovery of a G2 star, similar in type to our own Sun.[11] It is thought to be the companion star that contributed mass to the white dwarf that ultimately resulted in the supernova. A subsequent study, published in March 2005, revealed further details about this star: labeled Tycho G, it was probably a main-sequence star or subgiant prior to the explosion, but had some of its mass stripped away and its outer layers shock-heated from the effects of the supernova. Tycho G's current velocity is perhaps the strongest evidence that it was the companion star to the white dwarf, as it is traveling at a rate of 136 km/s, which is more than four times faster than the mean velocity of other stars in its stellar neighbourhood. This find has been challenged in recent years. The star is relatively far away from the center and does not show rotation which might be expected of a companion star.

In September 2008, the Subaru telescope obtained the optical spectrum of Tycho Brahe's supernova near maximum brightness from a scattered-light echo.[1] It has been confirmed that SN 1572 belongs to the majority class of normal SNe Ia.

An X-ray source designated Cepheus X-1 (or Cep X-1) was detected by the Uhuru X-ray observatory at 4U 0022+63. Earlier catalog designations are X120+2 and XRS 00224+638. Cepheus X-1 is actually in the constellation Cassiopeia, and it is SN 1572, the Tycho SNR.[12]